Ab initio investigations of the phase stability in group IVB and VB transition metal carbides

Using ab-initio calculations, the stability of various stable and metastable group IVB and VB transition metal carbide compounds at ambient pressure and 0 K were investigated. By comparing the formation enthalpy, vacancy diffusion activation energy and generalized stacking fault energy of these phases, the thermodynamic competition between vacancy-ordered and stacking-faulted phase formation mechanisms are seen among the carbides. In general, the group IVB carbides have a higher vacancy migration energy than that of the group VB carbides which could explain the difficulty in experimentally forming such vacancy-ordered phases. In addition, the group IVB carbides do not have a stable intrinsic stacking fault (ISF), which is present in the group VB carbides. This ISF is used to rationalize the absence of such faulted phases in the group IVB carbides but their presence in the group VB carbides.